Project Summary/Abstract Learning and predicting dangerous environments is crucial for individual fitness. Social fear learning is a process by which individuals can learn about harmful stimuli through observing the aversive situations of other conspecifics. Observational fear (OF) is a model of social fear learning where an observer witnesses a demonstrator?s aversive situation and responds by engaging in fear-related behaviors. For example, mice learn to fear a previously neutral context by watching a demonstrator mouse receive foot-shocks. Humans learn to fear a previously neutral conditioned stimulus (CS) by watching a demonstrator receive a shock when the CS is present. For both rodents and primates, OF is enhanced when the observer has prior experience in a similar aversive context, and when the observer is familiar with the demonstrator. However, the neural mechanisms that underlie how prior similar experience and social familiarity affect OF are currently unknown. The goal of this proposal is to investigate the underlying neural mechanisms that explain how prior similar experience and familiarity with the demonstrator facilitate OF in observers. The dorsal anterior cingulate cortex (ACC) has been established as an important center for OF. In mice, inhibition of ACC reduces OF, whereas stimulation of ACC facilitates OF. In humans, ACC activity is associated with enhanced OF. The ability of prior similar experience and social familiarity to enhance OF suggests that there is also a strong memory system component of OF. Specifically, dorsal hippocampus (dHPC) is critical for the formation and recall of contextual fear experience, whereas ventral HPC (vHPC) is critical for social recognition and discrimination. Moreover, afferents from dHPC through medial entorhinal cortex, and vHPC project to ACC. Thus, interactions between dHPC, vHPC, and ACC likely facilitate the effects of prior similar experience and social familiarity to enhance OF. This proposal will test the hypothesis that dHPC activity during prior shock experience contributes to the formation of a fear memory in ACC of observers that is reactivated during OF testing with a familiar demonstrator, thus enhancing OF. Aim 1 will establish and characterize an OF model that incorporates prior similar experience and social familiarity in male and female mice. Aim 2 will use chemogenetic methods to inhibit dHPC, vHPC, and ACC in observers during prior shock experience or during OF with a familiar demonstrator, to determine the necessity of these brain regions during prior shock experience and during OF testing to enhance OF. Aim 3, will combine chemogenetics and in vivo calcium imaging to image ACC neuronal activity in observers while dHPC is inhibited during prior shock experience or while vHPC is inhibited during OF testing with a familiar demonstrator. Together, the studies in this proposal will advance understanding about the fundamental neural mechanisms of social fear learning and empathy. Furthermore, the comprehensive training plan outlined in this fellowship award will provide extensive training in state-of-the-art neuroscience techniques, specifically chemogenetics and in vivo calcium imaging, in a strong institutional environment with plentiful training resources.